A particular application of the invention is in the control of transmission timings in base stations in UMTS (Universal Mobile Telecommunication System) type third generation cellular networks standardized by the organization 3GPP (3rd Generation Partnership Project). The invention is described hereinafter in its application to a UMTS network in Frequency Division Duplex (FDD) mode, and FIG. 1 shows the architecture of such a network.
The mobile service switches 10, belonging to a Core Network (CN), are linked to one or more fixed networks 11 and, by means of an interface called lu, to control units 12, or RNCs (Radio Network Controllers). Each RNC 12 is linked to one or more radio stations 13 by means of an interface called lub. The radio stations 13, distributed over the network coverage area, can communicate by radio with mobile terminals 14, 14a and 14b, called UEs (User Equipment). The radio stations can be grouped together to form nodes called “Nodes B”. Some RNCs 12 can additionally communicate with each other by means of an interface called lur. The RNCs and the radio stations form an access network called UTRAN (UMTS Terrestrial Radio Access Network). The UTRAN includes elements from layers 1 and 2 of the ISO model with a view to providing the links required on the radio interface (called Uu), and a Radio Resource Control (RRC) stage 15A belonging to layer 3, as described in the technical specification 3β TS 25.301, “Radio Interface Protocol Architecture”, version 4.2.0 published in December 2001 by the 3GPP. Viewed from the upper layers, the UTRAN acts simply as a relay between the UE and the CN.
FIG. 2 shows the RRC stages 15A, 15B and the stages of the lower layers that belong to the UTRAN and to a US. On each side, layer 2 is subdivided into a Radio Link Control (RLC) stage 16A, 16B and a Medium Access Control (MAC) stage 17A, 17B. Layer 1 includes a coding and multiplexing stage 18A, 18B. A radio stage 19A, 19B provides for the transmission of radio signals based on symbol trains supplied by stage 18A, 18B, and provides for the reception of signals in the other direction.
There are various ways of adapting the protocol architecture according to FIG. 2 to the UTRAN hardware architecture according to FIG. 1, and in general various structures can be adopted according to the channel types (see section 11.2 of the technical specification 3β TS 25.401, “UTRAN Overall Description”, version 4.2.0 published in September 2001 by the 3GPP). The RRC, RLC and MAC stages are in the RNC 12. Layer 1 is for example in the Node B. Part of this layer may however be in the RNC 12.
Layers 1 and 2 are each controlled by the RRC sub-layer, the characteristics of which are described in the technical specification 3β TS 25.331, “RRC Protocol Specification”, version 4.1.0 published in June 2001 by the 3GPP. The RRC stage 15A, 15B supervises the radio interface. It additionally handles flows to be transmitted to the remote station according to a “control plane”, as opposed to the “user plane” which corresponds to the handling of user data coming from layer 3.
UMTS in FDD mode supports a macrodiversity technique which involves anticipating that a UE can communicate simultaneously with separate radio stations in a so-called “Active Set” such that, in the downlink direction, the UE receives the same information several times and that, in the uplink (UL) direction, the radio signal transmitted by the UE is picked up by the radio station to form various estimations which are then combined in the UTRAN.
Macrodiversity results in a receive gain which improves the performance of the system owing to the combination of different observations of the same item of information. It also enables Soft Handovers (SHOs) to be achieved as the UE moves.
In macrodiversity, branching of transport channels for multiple transmission from the UTRAN or the UE and the combination of these transport channels in receive mode are operations for which a selection and combination module belonging to layer 1 is responsible. This module is at the interface with the MAC sub-layer, and it is located in the RNC serving the UE. If the radio stations involved depend on different RNCs communicating over the lur interface, one of these RNCs acts as SRNC and the other as DRNC.
When several RNCs are involved in a communication with a UE, there is generally one Serving RNC (SRNC), in which the layer-2-based modules (RLC and MAC) are located, and at least one Drift RNC (DRNC) to which a radio station is linked, and with which radio station the UE is in radio communication. Suitable protocols provide the exchanges between these RNCs over the lur interface, for example ATM (Asynchronous Transfer Mode) and AAL2 (ATM Adaptation Layer No. 2).
These same protocols can also be employed on the Sub interface for exchanges between a Node B and its RNC. Above the ATM and AAL2 layers, a Frame Protocol (FP) is used in the user plane to enable the SRNC to communicate with the Node B or Nodes B involved in a communication with a given UE.
This FP protocol is described in the technical specifications 3β TS 25.427, “UTRAN lub/lur Interface User Plane Protocol for DCH Data Streams”, and 3β TS 25.435, “UTRAN Tub Interface User Plane Protocols for Common Transport Channel Data Streams”, versions 4.3.0, published in December 2001 by the 3GPP. In particular, it provides signalling frames allowing transport channels to be synchronized in the manner described in section 7 of the technical specification 3β TS 25.402, “Synchronization in UTRAN Stage 2”, version 4.3.0, published in December 2001 by the 3GPP.
The objective of this transport channel synchronization is to obtain a layer 2 common frame numbering between the UTRAN and the UE, achieved using an 8-bit Connection Frame Number (CFN), managed by layer 2 for each Transport Block Set (TBS) exchanged with the UE by incrementing it by one unit every 10 ms.
This CFN is not transmitted over the air interface, but it is added to the frames exchanged over the lub interface. The physical layer maps it to a frame numbering kept up-to-date for each cell, defined by a System Frame Number (SFN) coded on 12 bits. The Node B increments this SFN at each new 10 ms radio frame and broadcasts it over the common control channels of the cell.
For a given TBS and a given cell, the offset between the CFN and the SFN is determined before the radio link between the Node B and the UE concerned is set up, in terms of an offset expressed by an integer number of frames (Frame Offset).
When a UE is not in soft handover, the cell included in its active set is considered as a so-called “reference cell”. The cell, which is selected as a reference cell, remains as a reference cell even if other cells are added to the active set.
UMTS is a radio communication system using Code-Division Multiple Access (CDMA), that is to say the symbols transmitted are multiplied by spreading codes consisting of samples known as “chips” whose rate (3.84 Mchip/s in the case of UMTS) is greater than that of the symbols transmitted. The spreading codes distinguish between various physical channels PhCH which are superimposed on the same transmission resource constituted by carrier frequency. The auto- and cross-correlation properties of the spreading codes enable the receiver to separate the PhCHs and to extract the symbols intended for it. For UMTS in FDD mode on the downlink (DL), a scrambling code is allocated to each base station, and various physical channels used by this base station are distinguished by mutually orthogonal “channelization” codes. For each PhCH, the global spreading code is the product of the “channelization” code and the scrambling code of the base station. The spreading factor (equal to the ratio of the chip rate to the symbol rate) is a power of 2 lying between 4 and 512. This factor is chosen as a function of the bit rate of the symbols to be transmitted on the PhCH.
The various physical channels obey a specific frame structure in the FDD mode, and 10 ms frames follow one another on the carrier frequency used by the base station. Each frame is subdivided into N=15 time slots of 666 μs. Each slot can carry the superimposed contributions of one or more physical channels, comprising common channels and dedicated channels DPCH (“Dedicated Physical CHannel”). The downlink DPCH can be seen as amalgamating a physical channel dedicated to control, or DPCCH (“Dedicated Physical Control CHannel”), and a physical channel dedicated to the data, or DPDCH (“Dedicated Physical Data CHannel”).
For the purpose of synchronizing its transmission timings, a UE chooses a reference radio link (RL), i.e. the first detected path (in time) of the corresponding downlink DPCCH/DPDCH frame from its reference cell. The uplink DPCCH/DPDCH frame transmission takes place approximately T0 chips after the reception of the reference RL (for an example value for T0, see section 7.6.3 “Uplink/downlink timing at UE” of the technical specification 3GPP 25.211, “Physical channels and mapping of transport channels onto physical channels (FDD) (Release 6)”, v 6.3.0, published in December 2004 by the 3GPP, which mentions that T0 is a constant defined to be 1024 chips). That is, the reference point for the UE initial transmit timing control is the reception time of the reference RL plus T0 chips. As the reception timing of this reference RL may drift over time, the UE has the capability to monitor and compensate for such a drift. However, this is a slow process as the UE should be capable of changing its transmission timing according to the received downlink DPCCH/DPDCH frame with a maximum adjustment rate of ¼ chip per 200 ms (see section 7.1 “UE Transmit Timing” of the technical specification 3GPP TS 25.133, “Technical Specification Group Radio Access Network; Requirements for support of radio resource management (FDD) (Release 6)”, v 6.8.0, published in December 2004 by the 3GPP).
In a mobility situation, should the reference RL be removed from the Active Set, the UE simply selects another reference RL in the Active Set.
A first scenario illustrating a problematic case arising from UE movements is exposed hereinafter:
A first radio link RL1 is established on a first, reference cell Cell1 controlled by a first base station Node-B1. The first slope on FIG. 3 indicates that the UE is moving away from the base station Node-B1 and thus the propagation delay on the RL1 increases. As explained above, the UE ensures that the uplink DPCH Transmission time is as close as possible to T0=1024 chips after the reception of the downlink DPCH from the cell Cell1. As mentioned above, this is done by small adjustments (no more than ¼ chips every 200 ms). At instant referenced t3 on FIG. 3, a second RL (RL2) is then added on a second cell Cell2 controlled by a second base station Node-B2. It is assumed that the measurements provided by the UE (based on observed time difference between its own timing and the second cell (Cell2) timing—see the definition of synchronization parameters “OFF” and “Tm” in Chapter 5, “Synchronisation Counters and Parameters” of the technical specification 3GPP TS 25.402, “Technical Specification Group Radio Access Network; Synchronisation in UTRAN Stage 2 (Release 6)”, v. 6.1.0, published in December 2004 by the 3GPP) are such that after rounding of the Frame Offset+Chip Offset parameters values provided to the second base station (Node-B2), the downlink DPDCH/DPCCH from the second cell Cell2, i.e. corresponding to the second radio link (RL2) is received at T0+αcell2 (t3) where αcell2 is between −148 and +148 chips. In this example, by rounding the Chip Offset to the nearest 256 chips-boundary, the transmission timing for RL2 is such that it is received at e.g. T0+αcell2 (t3)=T0+125 chips.
The second slope indicates that the UE is moving closer to the second base station Node-B2 and thus the propagation delay on the RL2 decreases. As the UE continues to move away from the first base station Node-B1, the propagation delay on the RL1 continues to increase and the UE continues to shift the UL DPCH Transmission time to ensure that it is as close as possible to 1024 chips after the reception of the downlink DPDCH/DPCCH from the first cell Cell1 (Cell1 is still its reference cell). As the UE continues to move closer to the second base station Node-B2, the propagation delay on the RL2 continues to decrease. This, added to the shifting of the UL DPCH Transmission time, means that the Reception instant of the DL DPCH from the second cell Cell2 is moving away from the UL DPCH Transmission time, i.e. T0+αcell2 is increasing. Then, at instant referenced t5 on FIG. 3, a third radio link RL3 on a third cell Cell3 controlled by the second base station Node-B2 is added to the Active Set. At this point in time, T0+αcell2 (t5)=T0+132 chips for the second radio link RL2. Based on measurement provided by the UE on observed time difference between its own timing and the third cell Cell3 timing, the SRNC will provide timing instructions to the second base station Node-B2 which will result according to mechanisms specified in 3GPP TS 25.402 in a third radio link RL3 on which the transmission occurs 256 chips later than on the second radio link RL2: T0+α3 (t5)=T0−124 chips. Therefore, this will create a situation in which two radio links (RL2 and RL3) transmitted by the same base station Node-B2 will be non simultaneous, since they will be transmitted approximately 256 chips apart.
A second scenario illustrating a problematic case arising from a change of reference cell for a UE is exposed hereinafter:
This scenario is illustrated by FIGS. 4-9 which shows the evolution (with time) of the DL DPCH reception time and UL DPCH transmission time, and FIG. 10 which shows message flows between a network controller (SRNC), its controlled base stations (Node-B1, Node-B2, Node-B3) and a UE.
First step (FIG. 4): A first radio link (RL1) is established on a reference cell (Cell1) controlled by a first base station (Node-B1). The reference cell (Cell1) provides a reference for the determination by the UE of the frame timing of the uplink DPCH transmission (see section 7.1.2 of the technical specification 3GPP TS 25.133). As illustrated on FIG. 4, a 148 chips window is defined around instant T0 such that UTRAN starts the transmission of the downlink DPCCH/DPDCH for each new radio link at a frame timing such that the frame timing received at the UE will be within T0+/−148 chips prior to the frame timing of the uplink DPCCH/DPDCH at the UE (see section 4.3.2.4, “Synchronisation procedure B” of the technical specification 3GPP TS 25.214
On FIG. 10, this first step is illustrated by the radio link setup request (RL-SETUP-REQ) and response (RL-SETUP-RESP) NBAP messages exchanged between the first base station (Node-B1) and its serving controller (SRNC) for the establishment of the first radio link (RL1). FIG. 10 also shows the RRC messages exchanged between the SRNC and the UE for the purpose of establishing an RRC connection for the UE (RRC CONNECTION SETUP) and the corresponding confirm from the UE that an RRC connection is established (RRC CONNECTION COMPLETE).
Second step (FIG. 5): A second radio link (RL2) is established on a second cell (Cell2) controlled by a second base station (Node-B2). The reference cell (Cell1) still provides a reference for the determination by the UE of the frame timing of the uplink DPCH transmission. It is assumed that the measurements provided by the UE (based on observed time difference between its own timing and the second cell (Cell2) timing—see the definition of synchronization parameters “OFF” and “Tm” in Chapter 5, “Synchronisation Counters and Parameters” of the technical specification 3GPP TS 25.402) are such that after rounding of the Frame Offset+Chip Offset parameters values provided to the second base station (Node-B2), the second radio link (RL2) is received at: T0+αcell2 (tstep2)=T0+125 chips before the UL DPCH frame timing at the UE, that is close to the upper bound of the receiving window (T0+/−148 chips).
On FIG. 10, this second step is illustrated by the radio link setup request (RL-SETUP-REQ) and response (RL-SETUP-RESP) NBAP messages exchanged between the second base station (Node-B2) and its serving controller (SRNC) for the establishment of the second radio link (RL2). FIG. 10 also shows the RRC messages exchanged between the SRNC and the UE for the purpose of adding the second radio link (RL2) in the active set of the UE (ACTIVE SET UPDATE) and the corresponding confirm from the UE that the active set update is completed (ACTIVE SET UPDATE COMPLETED).
Third step (FIG. 6): The first radio link (RL1), established on the reference cell, is removed from the Active Set. The UE then starts adjusting its transmit timing and chooses in an implementation-specific manner the new reference cell for determination of the UL DPCH frame timing at the UE. In this particular case, as there is only one radio link, the second radio link (RL2), in the Active Set, the UE chooses the second cell (Cell2) as its new reference cell.
The UE then starts to adjust the UL DPCH frame timing so that it the UL DPCH transmission time is as close as possible to 1024 chips after the reception of the DL DPCH from the second cell (Cell2) (new reference cell). This can be done only by small adjustments (no more than ¼ chips every 200 ms). Thus, T0+αcell2 (t) starts to progressively decrease with time.
On FIG. 6, the dashed lines indicate the position the uplink transmit instant and the receive window in the UE at tstep2 (second step), and the plain lines illustrate the positions at t3 (third step).
In this example, T0+αcell2 (tstep3)=T0+105 chips before the UL DPCH frame timing at the UE.
On FIG. 10, this third step is illustrated by the radio link deletion request (RL-DELETION-REQ) and response (RL-DELETION-RESP) NBAP messages exchanged between the first base station (Node-B1) and its serving controller (SRNC) for the deletion of the first radio link (RL1). FIG. 10 also shows the RRC messages exchanged between the SRNC and the UE for the purpose of deleting the first radio link (RL1) in the active set of the UE (ACTIVE SET UPDATE) and the corresponding confirm from the UE that the active set update is completed (ACTIVE SET UPDATE COMPLETED).
Fourth step (FIG. 7): A third radio link (RL3) is established on a third cell (Cell3) controlled by a third base station (Node-B3). It is assumed that the measurements provided by the UE (based on observed time difference between its own timing and the third cell (Cell2) timing—see the definition of synchronization parameters “OFF” and “Tm” in Chapter 5, “Synchronisation Counters and Parameters” of the technical specification 3GPP TS 25.402) are such that after rounding of the Frame Offset+Chip Offset parameters values provided to the second base station (Node-B3), the third radio link (RL3) is received at: T0+αcell3 (tstep4)=T0−115 chips before the UL DPCH frame timing at the UE, that is close to the lower bound of the receiving window (To+/−148 chips). Furthermore, as the UE continued adjusting the UL DPCH frame timing so that the UL DPCH transmission time is as close as possible to 1024 chips after the reception of the DL DPCH from its new reference cell (Cell2), T0+αcell2(t) has further decreased and is assumed to have reached the value (on FIG. 7): T0+αcell2 (tstep4)=T0+85 chips before the UL DPCH frame timing at the UE.
On FIG. 7, the dashed lines indicate the position of the uplink transmit instant and the receive window in the UE at tstep2 (second step), the plain lines indicate the positions at tstep4 (fourth step).
On FIG. 10, this fourth step is illustrated by the radio link setup request (RL-SETUP-REQ) and response (RL-SETUP-RESP) NBAP messages exchanged between the third base station (Node-B3) and its serving controller (SRNC) for the establishment of the third radio link (RL3). FIG. 10 also shows the RRC messages exchanged between the SRNC and the UE for the purpose of adding the third radio link (RL3) in the active set of the UE (ACTIVE SET UPDATE) and the corresponding confirm from the UE that the active set update is completed (ACTIVE SET UPDATE COMPLETED).
Fifth step (FIG. 8): The UE keeps adjusting the UL DPCH frame timing so that the UL DPCH transmission time is as close as possible to 1024 chips after the reception of the DL DPCH from its new reference cell (Cell2). T0+αcell2(t) has further decreased and is assumed to have reached the value (on FIG. 8): T0+αcell2 (tstep5)=T0+75 chips before the UL DPCH frame timing at the UE. In the same manner, since T0 has moved by 10 chips towards the reception instant of the DL DPCH from its new reference cell (Cell2), T0+αcell3(t) has decreased and is now assumed to have reached the value (on FIG. 8): T0+αcell3 (tstep5)=T0−125 chips before the UL DPCH frame timing at the UE.
On FIG. 8, the dashed lines indicate the position of the uplink transmit instant and the receive window in the UE at tstep2 (second step), the plain lines indicate the positions at tstep5 (fifth step).
Sixth step (FIG. 9): A fourth radio link (RL4) is established on a fourth cell (Cell3) controlled by the third base station (Node-B3). It is assumed that the measurements provided by the UE (based on observed time difference between its own timing and the fourth cell (Cell4) timing—see the definition of synchronization parameters “OFF” and “Tm” in Chapter 5, “Synchronisation Counters and Parameters” of the technical specification 3GPP TS 25.402) are such that after rounding of the Frame Offset+Chip Offset parameters values provided to the third base station (Node-B3), the fourth radio link (RL4) is received at: T0+αcell4 (tstep6)=T0+121 chips before the UL DPCH frame timing at the UE, that is close to the upper bound of the receiving window (T0+/−148 chips).
The UE keeps adjusting the UL DPCH frame timing so that the UL DPCH transmission time is as close as possible to 1024 chips after the reception of the DL DPCH from its new reference cell (Cell2). T0+αcell2(t) has further decreased and is assumed to have reached the value (on FIG. 9): T0+αcell2 (tstep6)=T0+65 chips before the UL DPCH frame timing at the UE. In the same manner, since T0 has moved by 10 chips towards the reception instant of the DL DPCH from its new reference cell (Cell2), T0+αcell3(t) has decreased and is now assumed to have reached the value (on FIG. 9): T0+αcell3 (tstep6)=T0−135 chips before the UL DPCH frame timing at the UE, that is very close to the lower bound of the receiving window.
On FIG. 10, this sixth step is illustrated by the radio link setup request (RL-ADDITION-REQ) and response (RL-ADDITION-RESP) NBAP messages exchanged between the third base station (Node-B3) and its serving controller (SRNC) for the addition of the fourth radio link (RL4) in the active set. FIG. 10 also shows the RRC messages exchanged between the SRNC and the UE for the purpose of adding the fourth radio link (RL4) in the active set of the UE (ACTIVE SET UPDATE) and the corresponding confirm from the UE that the active set update is completed (ACTIVE SET UPDATE COMPLETED).
This sixth step results in a situation in which two radio links, the third (RL3) and fourth (RL4) ones, should be transmitted with timings that are distant from a multiple of 256 chips. Both radio links are transmitted by the same base station (Node-B3).
However, some base station products feature the advantageous capability to perform substantially simultaneous transmission of radio links, for the purpose of optimized performances, efficiency, in particular with regard to power consumption and radio resource management, which leads to lower cost.
The two above-described scenarios provide examples where the transmission timings as currently specified by the 3GPP organization for UMTS FDD system does not allow the use of such an optimum capability.